Color-managed sequence gets brighter when changing frame size?

Ok, a little education.

First, toss the use of the ancient "gamma comp'' LUT, as there is a more sensible workaround now. That is, if you want to make an image that looks sorta like you want on only Macs without Reference modes that are set to HDTV. But is too dark and contrasty on all broadcast compliant systems, Macs with Reference modes set to HDTV, and nearly all Android devices, TVs, etc.

In the Color Management section of the Lumetri panel, the Settings tab, the tab named Settings ... set the Viewer Gamma to 1.96/Qt. Then just work the file as you want, and export without any LUT applied. The export will look very much like you in-Premiere view if viewed on QuickTime Player, or Chrome or Safari browsers. Like unto the old LUT thing.

Of course, it will be dark and contrasty/oversaturated in VLC and Potplayer, on Macs with Reference modes that are set to HDTV (so they use the correct display gamma for Rec.709), broadcast compliant systems, TVs, Android ... about everything else.

Because the issue is caused by Apple choosing to use a non-standard display gamma for Rec.709 video but only on their 'prosumer' Macs.

The full-on heavy duty Macs have screens with the Reference modes, with the option for HDTV, which uses the correct Rec.709/Bt.1886 display transform used by everybody but the Macs without Reference modes.

So your choice ... is your choice. Just understand exactly what you are choosing. As you cannot make a file that will look the same (well as much as is possible) on both Macs without Reference modes and everything else.

But setting Viewer Gamma to 1.96, then doing your color correction to your liking, is a vastly better replacement for the gamma comp LUT than using that LUT.

Update: I find that even just opening and saving the sequence settings has the same effect.